The extrusion of synthetic resin materials can be used for many purposes and, in general, the thermoplastic material, in the form of a granulate (e.g. a powder or other collections of flowable particles) is introduced into one end of an extruder having at least one worm which is rotated to displace the synthetic resin material axially in a worm cylinder so that the extruded material can emerge from the opposite end of this cylinder. Within the cylinder, while undergoing axial displacement in the manner described, the synthetic resin material also is subjected to a vigorous mechanical action involving shear and compression as well as mixing and homogenization so that the mechanical action by the generation of heat alone or in combination with external heating results in a plastification and liquefaction of the thermoplastic material.
The discharge end of the extruder can be provided with an aperture from which the material emerges with a certain wall thickness and this extruder length can be formed with a die to impart any desired shape to the synthetic resin material which emerges. The preferred form is a tube or pipe.
Such extruded strands can be subjected to blowing, i.e. the expansion by inflation with air, to draw the wall of the tube into a foil. This process is frequently termed foil blowing. A tubular strand can also form a parison for blow molding.
The wall of the tube can be left intact if the continuous strand emerging from the discharge end of the extruder is permitted to cool and the resulting solidified or hardened strand can be subdivided into lengths. The lengths thus formed can be utilized as pipes or can be employed for various purposes and can even be deformed or further shaped as desired.
One of the problems in the extrusion of tubular strands of synthetic resin material is to maintain the wall thickness substantially constant with changes in the characteristics of the material, operating parameters and external conditions. For this purpose a measuring or metering cylinder can be provided at the upstream end of the extruder and can form an upright column of synthetic resin granulate to be fed to the latter by gravity. The level in this cylinder is detected by transducer means generating electrical pulses with a pulse interval determined by this level and which are compared with set-point values determined by the extruding rate and the weight of the synthetic resin strand per unit length, an error or correction scale resulting from this comparison serving to control the feed of the granulate to the measuring cylinder.
In German patent document No. 1,964,386 there is described a device for carrying out a process of the aforedescribed type. With this device the extrusion or discharge rate of the synthetic resin strand can be so modified that the wall thickness can be kept constant between predetermined limits.
The operation of this conventional system is based upon the fact that there is a linear relationship between the detected rate or feed of material to the extruder, i.e. the extruder intake and the rate of material discharged from the extruder, i.e. the extruder outflow. With the control system of this patent, by comparison with systems without wall thickness control in the aforedescribed manner, it is possible to achieve a significant saving in the synthetic resin material which is utilized, i.e. the consumption of this synthetic resin material is reduced.
Practical use of such systems has demonstrated that for the customary cross section of extruded synthetic resin strands, a mean wall thickness can be maintained with good approximation. However, the absolute wall thickness or, more accurately, the instantaneous wall thickness, can vary significantly from the mean wall thickness value and indeed these variations can have a significant band width, i.e. there can be substantial variations above and below the mean value of the wall thickness.
This poses serious problems, especially when the synthetic resin strand is to be utilized as a parison or is to be used in the formation of a parison for the blow-molding operation. In this application of the extruder product a significant drop in wall thickness, or even a small local region, may result in defects in the blow-molded product, while any significant thickening may adversely effect the blow-molding process and the quality of the product produced therein.
Consequently, in spite of the fact that the mean wall thickness can be held constant without difficulty with the conventional units, the variation in absolute wall thickness, the instantaneous wall thickness or local wall thickness, creates problems which must be solved for effective use of the extruded product.